The invention relates generally to frame synchronization in a display system, and more particularly to a method and apparatus for asynchronous frame synchronization in a video graphics circuit.
Many applications involve the display of video graphics information. The source of such video graphics information may be a computer, a video cassette recorder (VCR), a digitized television signal, etc. Video graphics signals from various sources are often processed by circuitry that produces output signals suitable for a display device. Such display devices can include television sets, which operate under a number of different television display standards, and monitors such as cathode ray tube (CRT) monitors that operate under a different set of standards. As such, the signal requirements for different display devices can vary significantly.
In order to satisfy the needs of a number of different users, circuits which provide video graphics outputs for display are often required to provide more than one type of output signal. For example, a video graphics circuit may produce a television encoded signal suitable for display on a television set, and also be required to provide a CRT encoded signal suitable for display on CRT-type display devices. Because the standards specifying the encoding techniques for television and CRT-type display devices differ significantly, maintaining a uniform frame period between multiple display outputs can be troublesome.
Even when the frame rate of one display output can be chosen to approximate the frame rate of another output, the two frame rates are, in most cases, asynchronous. If no forced synchronization is induced between these asynchronous signals, one of the outputs will produce a xe2x80x9crollingxe2x80x9d effect on its corresponding display. This is because the alignment of the two frame periods of the outputs will continue to diverge. For example, if the second output frame rate differs from the first output frame rate by five clock periods per frame, the end of the second output frame after the first frame period will be five clocks different then the end of the second output frame. At the end of the second frame period they will differ by ten clock periods, and at the end of the third frame period they will differ by fifteen clock periods. The xe2x80x9crollingxe2x80x9d effect produced by this divergence is unacceptable.
One solution to this is forced synchronization. In forced synchronization, one of the frame periods is chosen to be the dominant, or master period. The other frame period, or the slave frame period, is synchronized to the dominant period by forcing the slave period to begin at the same time as the master period for each frame. As such, the slave frame period may be forced to reset to the beginning of the next frame either prematurely, resulting in truncation, or after the time where the next frame should have been started, resulting in an effective lengthening of the current frame. Both truncation and lengthening produce a xe2x80x9ctearingxe2x80x9d effect on the display that is undesirable.
Therefore, a need exists for a method and apparatus for synchronizing asynchronous display frame periods such that rolling and tearing effects are avoided.